1. Field of the Invention
The invention generally relates to optical scanning apparatus for collecting the light rays of a scanning beam emerging from a scanned transparency. More specifically the invention relates to apparatus for collecting and separating a polychromatic scanning beam into a plurality of separate beams composed of different wavelengths and, especially, to such optical apparatus having means for suppressing the effect of scratches in the transparency.
2. Description Relative to the Prior Art
In color-separating apparatus for optically separating a beam of polychromatic light, the beam is ordinarily split into several components--red, green and blue--that are directed toward separate photosensitive targets. This is conventionally done by passing the beam through two or more partially reflecting mirrors (often referred to as dichroic mirrors) having optical interference layers with color-selective reflecting and transmitting properties. However, the band of wavelengths reflected by an interference layer is strongly dependent on the effective optical path taken by a ray through the layer as determined mainly by the angle of incidence of the impinging beam relative to the normal. Where two or more interference layers are applied to a mirror, as is frequently the case, the selective reflection effect is further affected by incident angle-shift as the path length is changed in varying degrees in the different layers. In either case, as the incident angle increases further from the normal, the spectral cut of the dichroic filter, i.e., as exemplified by the reflection curve, shifts toward progressively smaller wavelengths.
In addition, with an increasing angle of incidence, an undesirable polarization phenomenon occurs due to asymmetry in the response of the electric vector characterizing the light beam. The electric vector for each wavetrain in the light beam can be resolved into two components, one perpendicular to the plane of incidence (the perpendicular component) and one lying in this plane (the parallel component). With increasing angle of incidence the coefficient of reflection becomes greater for the perpendicular component and smaller for the parallel component, meaning that the perpendicular component is preferentially reflected. As a result the mean reflection caused by both components is color-shifted as the angle of incidence is increased. In the case of either effective optical path shift or polarization effect, undesirable color shifts occur across the images formed upon the photosensitive targets. These problems are discussed in greater detail in several journal articles: P. M. van Alphen, "Applications of the Interference of Light in Thin Films," Philips Technical Review vol. 19, 59-67, 1957/58; H. de Lang and G. Bouwhuis, "Color Separation in Colour-Television Cameras," Philips Technical Review vol. 24, 263-271, 1962/63; or R. L. Seddon, "Interference Filters for Colorimetric Applications," Optical Coatings, vol. 50, 153-162, 1974.
Apart from their use in dichroic filters, interference layers are used to form filters that provide narrowband radiation detection. As an example, radiation reconnaissance systems having a wide field of view include interference filters to provide sharp cut-on or cut-off for narrowband detection, e.g., to detect a particular type of laser beam illuminating the reconnaissance system. Optical arrangements have been suggested for accepting a wide field of rays and reducing their impingement angle upon the interference filter (see, for example, the combination of a hyperhemispherical lens and a fresnel lens described in U.S. Pat. No. 3,761,184 or the optical cone-like condenser described in U.S. Pat. No. 4,225,782).
In view of these well known problems with interference layers, a number of optical designs have been proposed to control polarization and angle shift characteristics in color scanners. For example, in using a flying spot scanner to form a raster scan upon a transparency, at least one condenser lens is usually inserted in the optical path to refract the beams emanating from points outside the middle of the raster towards the axis of the system. This is done in such a way that the axes of most beams reach the dichroic mirrors at a similar angle irrespective of their point of origin on the raster (see H. van Ginkel, "Flying-Spot Scanners for Colour Television" Philips Technical Review, vol. 21, 1959/60, pp. 234-250). Another optical design is based on the Philips color-separating prism system described in the above-cited de Lang and Bouwhuis article in the Philips Technical Review. This prism system utilizes a compact combination of interference layers cemented between faces of prisms and small air gaps between selected sets of prisms. The Philips optical geometry in combination with glass prisms allows the angles of incidence to be reduced over what can be obtained with conventional open air plate type color separation systems.
However neither the condenser lens nor the prism system are sufficiently effective regarding widely diverging rays. Particularly in the case of a transparency, light-scattering artifacts (such as scratches, dust particles, and the like) are common sources of widely diverging rays. A typical artifact is a scratch on the transparency which scatters light from a scanning beam. Oftentimes, some of the scattered light will be at such an extreme angle that it will not be collected at all by the light collection apparatus. In these cases, the signal to the photosensitive targets will be less than that for areas immediately adjacent to the scratch that contain the same scene detail. Where a reproduction is made from such target signals, the scratch will be readily visible because of the decreased signal. In the case of transparency scanners it has been suggested to surround the optical system with reflecting surfaces to redirect light scattered by scratches back upon the photosensitive targets to obscure the effect of scratches in the transparency.
Other less extremely angled rays of scattered light may enter the light collection apparatus but still at a sharply increased angle of incidence relative to the interference layers. Since the reflection characteristics of the interference layers are modified by angle-shift effects, some rays of the scratch-scattered beam are deflected toward incorrect targets. In the case of color reproductions, the scratch then appears in a different color from the adjacent areas. The reflector-encased design suggested above does not inherently control angle-shift, therefore leading to the appearance of color-shifted scratches even though proper neutral exposure may be achieved for the area of the reproduction corresponding to the scratched area of the transparency.
Illumination of the transparency with strongly diffuse light transmitted by a diffuser would help to suppress the effect of scratches in the transparency. However, because it would fail to provide adequate color separation and lead to a serious reduction in signal-to-noise ratio for the targets, it is basically impractical where a specular scanning source is required. Another method has been to use a "liquid gate" in which the transparency faces are coated with a liquid layer which smoothes out the surface and renders dirt and scratches much less visible. However, the attendant mechanical and operational problems of a "liquid gate" limit its practical applications. In yet another approach, United Kingdom Patent Specification No. 1409153 describes the procedure of detecting light scattered from blemishes on cine film in order to electrically substitute a grey level or adjacent picture signal for the scanning signal obtained from the blemished area. Besides the circuit complexity of implementing such a procedure, the blemished area is incorrectly reproduced relative its original color and density.